Various methods are known to produce hydroxylated aromatic compounds. The majority of such processes require either the purchase or the formation of an aromatic compound bearing a substituent besides a hydroxyl group. That preexisting substituent then is converted to a hydroxyl group. Direct hydroxylation of aromatic compounds theoretically should be more economical.
Known methods for directly hydroxylating aromatics—particularly for directly converting benzene to phenol—are gas phase processes. In such processes, benzene vapor is partially oxidized at high temperature, typically by reaction with nitrous oxide over a catalyst bed.
Gas phase direct conversion processes are less than ideal for a number of reasons. The energy required to supply the initial heat to begin the reaction is costly. In addition, the reaction of benzene and nitrous oxide is highly exothermic. Expensive, complex system designs may be required to handle the excess heat.
The expense of such reactions is further increased by coke formation from the decomposition products formed at such high temperatures. The average productivity of a catalyst for gas phase oxidation of benzene is only about 4 mmol phenol/g catalyst/hour. The coked catalyst must be regenerated at frequent intervals.
Finally, the reported selectivity of nitrous oxide to phenol in these gas phase processes is low. While selectivities of benzene to phenol of 97-98 mol % are reported, the reported selectivity of nitrous oxide to phenol is only about 85 mol %.
A more economical and efficient process is needed for directly hydroxylating benzene.